In the widely used T1 and similar trunking systems digital data is transmitted between switching offices at the rate of 1.544 MHz for transmission of PCM encoded voice and data. As in all similar data transmission systems, a clock signal must be recovered from the received data stream in order to regenerate and strobe the incoming digital pulses, degraded by the transmission path, prior to actual processing, switching or rerouting. Normally, the incoming digital cable terminates in an office repeater located in the switching office of the telephone company and which is adjusted to match the final stretch of unrepeatered transmission line.
While the office repeater delivers a regenerated data stream, it is still necessary to supply the receive section with both the incoming data stream and a separate stable and continuous clocking signal extracted from the data stream and coherent therewith. The equipment that performs this, among other functions, is generally known as the digital termination and, in the context of the 1.544 MHz transmission systems, as the digroup terminal equipment. The popular 1.544 MHz systems accommodate 24 voice channels, hence the term digroup, a group being 12 voice channels.
In the digroup terminal equipment a receive amplifier is followed by a clock recovery circuit which normally consists of some type of tuned circuit (an electronic fly-wheel) having a pass band centered around 1.544 MHz and having a sufficiently high quality factor Q. As the incoming digital data pulses excite the tuned circuit they cause it to "ring" due to its high Q and produce an oscillatory output even though the excitation may have ceased for a few clock durations during the transmission of zeros. Given the practical limitations of actual tuned circuits, it is necessary to excite a clock recovery circuit for at least once every sixteen clock periods in a 1.544 MHz system. Accordingly, the coding standards for such transmission systems call for a maximum of fifteen consecutive zeros in the transmitted data stream in order to maintain synchronization at all times.
The tuned circuit itself must have a minimum Q factor which also determines the resulting bandwidth, thereby defining the necessary center frequency accuracy of the circuit. The higher the Q (necessary for longer "ringing"), the narrower is the bandwidth and the more accurately must the circuit be tuned. In practical terms, it is necessary that the frequency determining components of the tuned circuit must have a tolerance below 1% over both life and temperature range of the circuit. This tolerance requirement is too stringent for practical components and devices and as a result the tuning circuit must be tuned and possibly retuned manually to the centre frequency. The initial tuning is normally done during production testing, with readjustment in the field if necessary.